Well for me personally, I want black hair, specifically because my character concept always called for it, but also because it never made sense to me that void elves didn't have black as a hair color option (and white too for that matter).
For other people, particularly those that have been campaigning for playable alliance high elves, void elves were a perversion of that request. Basically a corrupt-a-wish. No one ever asked for void elves to be a playable option because void elves didn't exist prior to allied races being added to the game (and more specifically don't exist until their unlock scenario), whereas the highmountain, nightborne, and lightforged did, and had foundational elements laid prior to allied races ever being a thing.
Since Blizzard added the blue eyes and broad range of skin options to void elves, and billed these as "high elf customization", the only thing left to finish the high elf aesthetic is a few natural hair color options. This is asked for by those seeking playable alliance high elves and also by those who just want a bigger range of colors for their their hair. The current hair palette is very "one-note" and more variety is desirable by many. Since alliance high elves as their own race is unlikely to ever happen since void elves exist and have high elf customization options, those seeking alliance high elves are simply trying to finish that aesthetic. And other players, such as myself, simply want more variety (and specifically black in my case).
Like it or not, Blizz basically set a precedent that high elf customization options are part of the void elf package when they added "high elf customization" to void elves, and asking for the last bit to finish the aesthetic is well within reason (and I agree with this). Blizzard also doesn't have to copy and paste all the blood elf hairstyles and colors to fulfill that request either. If Blizzard wants to add more hairstyles, they can make new ones or simply port styles from other alliance races. If Blizzard wants to make certain the void aesthetic is always an option they can turn the hair tentacles into a toggle like the night elf hair vines so that all void elf hairstyles can have tentacles if the player wants them to. And as for hair colors Blizzard could add a few token colors in the human range to basically fulfill the request.
This picture is one I like to reference. It adds options, both for the void aesthetic, and options that can also fulfill the high elf aesthetic, all without copying stuff from blood elves:
